Electromagnetic waves of various frequencies are emitted in an environment having many electromagnetic radiation sources, such as electronic equipment and communication equipment. It has been pointed out that the electromagnetic radiation from such equipment can cause neighboring devices to malfunction or the equipment itself can be caused to malfunction by any extraneous electromagnetic wave. Therefore, these types of equipment are required to have electromagnetic compatibility (hereinafter “EMC”) from the design and development phase.
With the advent of ubiquitous era, electronic equipment and communication equipment using microwaves and millimeter waves are increasing, and use of low-frequency bands has been being shifted to use of high-frequency bands. Examples of equipment using a high frequency band include fourth generation mobile phones (5 to 6 GHz), ultrafast wireless LAN (60 GHz), and automotive millimeter wave radars (77 GHz). Use of microwaves and millimeter waves is also beginning to increase in broader fields including a microwave energy transmission test in aviation/space projects or a high-power emitting radar system used in military-related applications.
While most of the frequencies set out in standards relating to electromagnetic interference (EMI) have been up to 1 GHz, the tendency toward use of high frequencies in electronic equipment and communication equipment has recently extended the frequencies to be used to about 18 GHz. It is predicted that manufacturers are obliged to perform EMI measurement in a still higher frequency range before the coming of a full-scale ubiquitous society.
From the viewpoint of safety of anechoic chambers, electromagnetic wave absorbers (hereinafter referred to as “wave absorber(s)” or simply “absorber(s)”) have recently come to be required to have incombustibility. In particular, an anechoic chamber in which high power is emitted as in the above described microwave energy transmission test or high power emitting radar system test, there is a possibility of a wave absorber producing heat on receipt of the radiation, suffering damage, and reducing its absorbing performance. It is also pointed out that an absorber can ignite, or a device being tested can produce heat and ignite to cause the absorber to catch fire.
From the background described, there is a need for a safe anechoic chamber for microrwaves and millimeter waves that allows EMC evaluation of a wide range of microwave and millimeter wave equipment from general electronic components to special high-power systems. Accordingly, there has been a demand for a wave absorbing material that exhibits excellent radiation absorbing performance in the microwave and millimeter wave range and withstands high power emission tests. A number of proposals have hitherto been made on wave absorbing materials.
For example, Japanese patent 4697829 proposes a carbon nanotube composite molded material composed of a matrix and carbon nanotubes aligned in the matrix along a given direction, the carbon nanotubes being uncovered with a ferromagnetic material. Compared with carbon nanotube composite material in which carbon nanotubes are randomly dispersed, the proposed material is described as having high electrical conductivity with a smaller amount of carbon nanotubes and, when applied to wave absorption, exhibits anisotropy and is therefore useful for wave absorption. However, it is difficult with the structure described in Japanese patent 4697829 to obtain excellent wave absorbing characteristics in the broad frequency band of microwaves and millimeter waves. According to Japanese patent 4697829, the contemplated matrix materials are organic materials, such as thermoplastic resins, curing resins, rubbers, and thermoplastic elastomers. It is unfeasible with such matrix materials to provide wave absorbers withstanding a high power emission test.
JP 2005-231931A discloses a cement-based wave absorber obtained by mixing flaky iron oxide particles having a predetermined average thickness and a predetermined minimum width in the main plane into an inorganic water settable material and molding the mixture into shape. The wave absorber is described as exhibiting stable absorption characteristics in a high frequency range of the order of gigahertz and being able to be designed to be thin and lightweight with a high dielectric constant and a high dielectric loss. The wave absorber is also described as being incombustible, free from deterioration by ultraviolet rays, and capable of withstanding long-term outdoor use.
Composed mainly of an inorganic material, the structure of the wave absorber of JP 2005-231931A is incombustible and excellent in power durability and is therefore preferred for use in high power emission tests. Nevertheless, because of the use of a magnetic material as a main component, the frequency band the wave absorber serves to absorb is limited to several tens of gigahertz. That is, the wave absorbent is incapable of achieving excellent wave absorbing performance over a broad microwave and millimeter wave range of from 1 to 110 GHz.
Thus, we have not yet acquired a wave absorber having excellent electromagnetic radiation absorbing characteristics in a broad microwave and millimeter wave range, incombustibility, and high power durability.